In recent years, there has been a growing demand for high-strength steel (high-tensile steel) that can contribute to weight reduction of structures in the fields of automobiles, household electrical appliances, and construction materials. In high-tensile steel techniques, the addition of Si to steel facilitates the manufacture of high-strength steel strips having high stretch flangeability, and the inclusion of Si or Al facilitates the manufacture of steel strips having retained γ and high ductility.
In a high-strength cold-rolled steel strip containing an easily oxidizable element, such as Si or Mn, however, the easily oxidizable element may be enriched on a surface of the steel strip during an annealing process and forms an oxide of the easily oxidizable element, such as Si or Mn, thereby causing poor surface appearance or poor chemical conversion treatability, for example, in phosphating.
In a hot-dip galvanized steel strip containing an easily oxidizable element, such as Si or Mn, the easily oxidizable element may be enriched on a surface of the steel strip during an annealing process and forms an oxide of the easily oxidizable element, such as Si or Mn, thereby impairing platability and forming an ungalvanized surface. The easily oxidizable element also reduces the alloying rate after plating. In particular, a SiO2 oxide film on a surface of a steel strip significantly impairs wettability between the steel strip and a hot dipping metal and retards the diffusion of ferrite and a plating metal in alloying. Thus, Si is particularly likely to impair platability and alloying treatability.
A method for avoiding this problem may be a method for controlling the oxygen potential in an annealing atmosphere.
For example, Patent Literature 1 discloses a method for adjusting the dew point in a later part of a heating zone and a soaking zone to a high dew point of −30° C. or more as a method for increasing the oxygen potential. This method has the advantage that the method produces some effect and it is industrially easy to adjust the dew point to the high dew point. However, the method has the disadvantage that it is not easy to manufacture the type of steel that is unsuitable for operation at a high dew point (for example, Ti-IF steel). This is because it takes a long time to change the annealing atmosphere from a high dew point to a low dew point. Furthermore, the method produces an oxidizing furnace atmosphere, and incorrect operation results in a pickup defect due to deposition of an oxide on a hearth roll or damage to furnace walls.
Another method may be a low oxygen potential method. However, since Si and Mn are highly oxidizable, it is very difficult to stably form an atmosphere having a low dew point of −40° C. or less in which oxidation of Si and Mn is suppressed in a large continuous annealing furnace in a continuous galvanizing line (CGL) or a continuous annealing line (CAL).
For example, Patent Literatures 2 and 3 disclose a technique for efficiently forming a low-dew-point annealing atmosphere. These techniques are applied to relatively small furnaces of one-path vertical furnaces and do not consider annealing of steel strips containing an easily oxidizable element, such as Si or Mn, in multipath vertical annealing furnaces, such as CGL and CAL.